1. Technical Field
This disclosure pertains to a loop filter and, more particularly, to a high ohmic loop filter with reduced noise that also facilitates IC integration comprising a first node for providing an input signal to the loop filter and a second node for providing an output signal for the loop filter, and further comprising a cascade arrangement of at least a first circuit for generating a zero, a second circuit for generating a first pole and a third circuit for generating a second pole to form a passive loop filter of at least third order. The cascade arrangement includes at least one signal path coupled between the first node and the second node. Such loop filter may be applied, for example, in a phase locked loop which may be applied in a frequency synthesizer, for example in communication or entertainment applications such as radio frequency tuners.
2. Description of the Related Art
Frequency synthesizers may be employed in communication or entertainment applications such as in receivers of radio frequency tuners for receiving and tuning broadcasting signals. A frequency synthesizer of such a system may include a phase locked loop (PLL) having an oscillator, such as a voltage-controlled oscillator, a loop filter, and a phase-frequency detector. The phase-frequency detector compares the phase and frequency of a periodic input signal against the phase and frequency of the oscillator. The output of the phase-frequency detector is a measure of the phase and frequency difference between the two inputs. Control signals from the phase-frequency detector are supplied to a charge pump, which generates a control signal (e.g., a current signal) that is low-pass filtered by a loop filter and then provided to the voltage-controlled oscillator. The voltage-controlled oscillator (VCO) generates the output signal of the PLL. This output signal can be used, for example, as local oscillator signal for a receiver mixer of a receiver chain in a tuner for radio frequency signals. As the VCO is driven by the loop filter, the loop filter determines loop characteristics of the PLL, such as the settling time and loop stability.
One approach that has been used for implementing a loop filter is shown in FIG. 1. The loop filter according to FIG. 1 is of third order, i.e., it includes a third order transfer function. The third order passive loop filter according to FIG. 1 includes a first node 1 for providing an input signal to the loop filter. For example, the input signal may be a current ICP provided by a charge pump of a PLL. The loop filter further includes a second node 2 for providing an output signal of the loop filter, which may be a control voltage VTUNE provided to a voltage-controlled oscillator of a PLL. For realizing the transfer function the loop filter further includes a cascade arrangement of RC-sections having a capacitor C1 and resistor R2 for generating a zero in the transfer function, a capacitor C2 and resistor R2 for generating a first pole, and a capacitor C3 and a resistor R3 for generating a second pole in the transfer function to form a passive loop filter of third order. The cascade arrangement is coupled in a signal path 3 between the first node 1 and the second node 2. The third order passive loop filter according to FIG. 1, therefore, includes in first order approximation an integrator with zero 1/(C1R2), a first pole 1/(C2R2), and a second pole 1/(C3R3).
According to FIG. 2, another approach of a loop filter that has been used is shown. In FIG. 2, the circuit structure according to FIG. 1 is extended to form a passive loop filter of N-th order by adding additional poles with additional RC-sections C4, R4 up to CN, RN in the cascade arrangement between first node 1 for receiving the input signal 6 and node 2 for providing the output signal VTUNE. For optimum steepness of the filter slope, loading of the k−1-th pole by the input impedance of the k-th pole with k=2 . . . N−1 must be avoided. Therefore, the condition 1<<(Rk/Rk-1)+(Ck-1/Ck) with k=3 . . . N must be fulfilled. When applied in a PLL, the charge pump current ICP and the loop filter impedance are usually chosen such that the noise contribution of the resistors R2, . . . , RN is acceptable. Hence, usually the higher the loop filter order is, the lower the loop filter impedance and the higher the charge pump current must be. Furthermore, the value of CN must be much larger than the input capacitance of the VCO, which can be quite large, to obtain a well controlled transfer function of the loop filter. During the so called pull-in transient time (which is determining the settling time of the loop filter until all capacitors are charged and stationary conditions of the loop filter are reached) the output signal VTUNE is slew rate limited (which is approximately ICP/2C1 as usually capacitor C1 is significantly greater than the other capacitors, i.e., C1>>C2 . . . CN).